1. Field of the Invention
The present invention relates to a data transmission system and a receiver unit therefor. More particularly, the present invention relates to a data transmission system which transports data between a transmitter unit and a receiver unit over a telephone subscriber loop, as well as to the receiver unit used in this system.
2. Description of the Related Art
In recent years, the use of multimedia communication services such as the Internet has become increasingly ubiquitous in our daily activities, both at work and at home. To meet such user requirements, there is an urgent need to provide highly reliable, cost effective digital access network systems. However, constructing a completely new telecommunication infrastructure for multimedia services is extremely costly and time consuming. This has motivated various proposals of new high-speed data communication techniques that use existing telecommunication networks. Digital Subscriber Lines (xDSL), for example, are known as one of the enabling technologies for high-speed data communications over plain old phone lines. This xDSL technology is a collection of signal transmission techniques using subscriber lines, and in another aspect, it is an advanced modulation/demodulation technique. Such XDSL techniques are broadly divided into two groups: ones that provide symmetric upstream and downstream transmission rates, and ones that provide asymmetric rates. Here, the term xe2x80x9cupstreamxe2x80x9d refers to the direction from a subscriber""s premises to its nearest central office, and the xe2x80x9cdownstreamxe2x80x9d refers to the opposite direction. The ITU-T recommendations for Asymmetric DSL (ADSL) include the following two versions: G.992.1 (G.dmt) for a downstream transmission rate of about 6 Mbps, and G.992.2 (G.lite) for about 1.5 Mbps. Both versions use Discrete Multitone (DMT) techniques for signal modulation.
(1) DMT Modulation System
Conventional DMT modulation techniques will be described below with reference to FIG. 14, focusing on the downstream signal modulation and demodulation in a G.lite-based ADSL system. The upper half of FIG. 14 shows a transmitter unit which comprises: a serial-to-parallel buffer 10, an encoder 20, an IFFT unit 30, a parallel-to-serial buffer 40, a D/A converter 50, and a transmission bit map 60. The serial-to-parallel buffer 10 stores transmission data for a single symbol period of 250 microseconds (i.e., the reciprocal of a symbol rate 4 kHz) and converts it into a parallel data format. The encoder 20 applies a prescribed modulation processing to the parallel data supplied from the buffer 10. The inverse fast Fourier transform (IFFT) unit 30 processes the output data of the encoder 20 using IFFT algorithms. The parallel-to-serial buffer 40 converts the transformed data back into a serial data format, as well as adding a cyclic prefix (described later) to each symbol. The digital-to-analog (D/A) converter 50 converts the serial data to an analog signal at a sampling rate of 1.104 MHz and outputs it to a metallic subscriber line 70. The transmission bit map 60 is an allocation table describing how many data bits should be assigned to each DMT carrier. This table is called the xe2x80x9cbit mapxe2x80x9d in the ADSL terminology.
The lower half of FIG. 14 shows a receiver unit, which comprises: an A/D converter 80, a TEQ unit 90, a buffer 100, an FFT unit 110, an FEQ unit 120, a decoder 130, a parallel-to-serial buffer 140, a reception bit map 150, and a TEQ training block 160. The analog-to-digital (A/D) converter 80 receives a DMT-modulated analog signal transmitted over the metallic subscriber line 70 and converts it into digital form at a sampling rate of 1.104 MHz. The time domain equalizer (TEQ) unit 90 then processes this digital data signal in a prescribed manner, so that intersymbol interference (ISI) to a cyclic prefix, which has been added at the parallel-to-serial buffer 40, will settle within the period of that cyclic prefix. The serial-to-parallel buffer 100 converts the output data of the TEQ unit 90 into parallel data, after removing a cyclic prefix from each symbol. The fast Fourier transform (FFT) unit 110 converts the output data of the serial-to-parallel buffer 100 into parallel data signals in the frequency domain by using FFT algorithms. The frequency domain equalizer (FEQ) unit 120 equalizes those frequency-domain data signals according to the transmission characteristics (or frequency response) of the metallic subscriber line 70. The decoder 130 demodulates the output data of the FEQ unit 120 in a prescribed manner. The parallel-to-serial buffer 140 receives parallel data from the decoder 130 and converts it into serial data. As the counterpart of the transmission bit map 60, the reception bit map 150 stores information about the number of data bits assigned to each carrier at the sending end. Based on this information, a decoder 130 and parallel-to-serial buffer 140 decodes the received data. The TEQ training block 160 adjusts the characteristics of the TEQ unit 90, with reference to the output signals of the FFT unit 110.
The above-described conventional system operates as follows. The transmitter unit accepts data to be transmitted at the serial-to-parallel buffer 10, which actually stores data bits for a single symbol period of 250 microseconds (i.e., the reciprocal of the symbol rate 4 kHz). Those stored data bits are divided into groups according to the bit allocation previously defined in the transmission bit map 60. They are then supplied to the encoder 20, which maps each given bit sequence to specific code points in the signal constellation of quadrature amplitude modulation. Those constellation points are passed to the IFFT unit 30. The IFFT unit 30 performs inverse fast Fourier transform to accomplish the quadrature amplitude modulation of each constellation point. The data signal modulated in this way is then output to the parallel-to-serial buffer 40. Note here that the DMT modulation is realized by a combination of the encoder 20 and IFFT unit 30.
The parallel-to-serial buffer 40 now chooses the 240th to 255th samples of the IFFT output data and adds a copy of those sixteen samples to the beginning of a DMT symbol. This is called the xe2x80x9ccyclic prefix,xe2x80x9d the details of which will appear in a later section. The symbol data with a cyclic prefix is now sent from the parallel-to-serial buffer 40 to the D/A converter 50. The D/A converter 50 converts it into an analog signal at the sampling rate of 1.104 MHz and transmits it toward the remote subscriber over the metallic subscriber line 70.
At the subscriber""s end, the A/D converter 80 converts the received signal to a digital data signal at the rate of 1.104 MHz and supplies it to the TEQ unit 90. This digital data signal has been impaired with intersymbol interference. The TEQ unit 90 processes it in such a way that the effect of intersymbol interference will be confined within a limited period of the 16-sample cyclic prefix. The processed signal is then stored in the serial-to-parallel buffer 100, the length of which is one DMT symbol period. The serial-to-parallel buffer 100 converts this signal into parallel form, removing a cyclic prefix from each symbol. The FFT unit 110 demodulates the resultant parallel data signals with fast Fourier transform algorithms, thereby reproducing the original constellation points. Those reproduced constellation points are then fed to the FEQ unit 120 to compensate for the amplitude and phase distortion that has occurred during the travel over the metallic subscriber line 70. This equalization is performed for individual carriers having different frequencies. The decoder 130 then decodes the equalized signals according to the reception bit map 150, which is identical to the transmission bit map 60. (More details will be provided in a later section, about the signal processing path from the TEQ unit 90 to the decoder 130.) Finally, the decoded data signals are stored into the parallel-to-serial buffer 140, and the resulting serial bit stream is output as fully decoded receive data. The FFT unit 110 also supplies constellation points to the TEQ training block 160 for training the TEQ unit 90. The TEQ training block 160 uses them to determine a set of parameters that characterize the TEQ unit 90.
(2) Crosstalk From TCM-based ISDN Services
The performance of an ADSL line is affected by ISDN signals transmitted in adjacent lines because of their coupling effects. This impairment is known as crosstalk interference, and the following problem (referred to herein as xe2x80x9cTCM crosstalkxe2x80x9d) will occur in the case of ISDN lines using a time compression multiplexing (TCM) technique, or xe2x80x9cping pongxe2x80x9d method.
TCM-based ISDN systems are designed to operate in synchronization with a 400 Hz timebase signal 210, as depicted in section (A) of FIG. 15. Downstream data is sent from the central office to the subscriber during the first half cycle of the timebase signal 210, while upstream data is sent from the subscriber to the central office during the second half cycle. For this reason, the ADSL equipment at the central office encounters near end crosstalk (NEXT) 220 from local ISDN transmitters during the first half cycle of the 400 Hz timebase signal 210, as well as suffering far end crosstalk (FEXT) 230 from upstream signals transmitted by the subscriber""s ISDN equipment during the second half cycle, as shown in section (B) of FIG. 15. The subscriber""s ADSL equipment, in turn, suffers FEXT 240 during the first half cycle of the timebase signal 210 and NEXT 250 during the second half cycle, as shown in section (C) of FIG. 15.
In this specification of the invention, those particular periods where the system suffers NEXT and FEXT interference are called xe2x80x9cNEXT periodsxe2x80x9d and xe2x80x9cFEXT periods,xe2x80x9d respectively. Generally, the severity of interference is higher in a NEXT period, compared to that in a FEXT period; part (D) of FIG. 15 indicates such NEXT and FEXT periods as viewed from the subscriber side.
(3) Sliding Window
The concept of xe2x80x9csliding windowxe2x80x9d has been introduced so as to provide a practical digital subscriber line system which can send ADSL signals with higher quality even in such an environment where the above-described TCM crosstalk is prevalent. The sliding window is used to identify the FEXT periods, in which the magnitude of noise interference is relatively small. By effectively utilizing such FEXT periods, the system can reliably transport data to the destination, with minimum interference of crosstalk noises.
Consider, for example, an ADSL signal being transmitted downstream from an office-side ADSL transmission unit (ATU-C) to a subscriber-side ADSL transmission unit (ATU-R). In this situation, the sliding window indicates the state of the ADSL signal as follows. That is, as shown in section (E) of FIG. 15, the sliding window 270 indicates that some symbols are within a FEXT period in their entirety, when viewed from the subscriber""s end. The ATU-C then sends those symbols to the ATU-R as xe2x80x9cinside symbols.xe2x80x9d The other symbols are included within a NEXT period in their entirety or in part, and thus the ATU-C sends them as xe2x80x9coutside symbols.xe2x80x9d This transmission technique is called the xe2x80x9cdual bit mapxe2x80x9d method, since two different bit maps are used to modulate and demodulate the inside and outside symbols. The same method applies to the upstream direction; the ATU-R at the subscriber""s end transmits DMT symbols according to the sliding window.
It should be noted here that there is another operation mode in which the ADSL system uses only one bit map in a FEXT period. This is called xe2x80x9csingle bit map mode,xe2x80x9d or xe2x80x9cFEXT bit map mode.xe2x80x9d During the period outside the sliding window, in FEXT bit map mode, the ATU-C transmits solely a pilot tone in the downstream direction for timing synchronization purposes, while the ATU-R transmits nothing in the upstream direction.
(4) Frame Structure
The concept of xe2x80x9chyperframexe2x80x9d has been introduced in order to provide a digital subscriber line system which can send ADSL signals with higher quality even in a TCM-crosstalk-prevalent environment described above. In the ADSL techniques, one frame corresponds to one symbol, and one superframe consists of 69 frames as shown in section (C) of FIG. 16. More specifically, those 69 frames 320 include 68 data frames and one special frame containing a synchronization symbol (S) in a normal communication session. As section (B) of FIG. 16 indicates, five superframes make a single hyperframe.
A synchronization frame may contain an inverse synchronization symbol (I), instead of the synchronization symbol (S) mentioned above. In the example shown in section (B) of FIG. 16, the fourth superframe carries an inverse synchronization symbol. FIG. 17 shows the difference between those two symbols. The inverse synchronization symbol (I) is 180-degree out of phase with the synchronization symbol (S) for DMT carriers other than the pilot tone, as shown in section (B) of FIG. 17. For the pilot tone, they are in phase with each other as shown in section (A) of FIG. 17. The inverse synchronization symbol (I) is inserted in this way, making it possible for the receiver to recognize which superframe it is receiving.
Referring back to part (B) of FIG. 16, a downstream ADSL hyperframe is transmitted from ATU-C to ATU-R. In this case, the ADSL specification stipulates that an inverse synchronization symbol (I) be placed at the fourth superframe in a hyperframe. In contrast to this, the specification requires that the upstream hyperframe should contain an inverse synchronization symbol in its first superframe. Section (A) of FIG. 16 shows a 400 Hz timebase signal 310 used by the aforementioned TCM-based ISDN services. The ADSL hyperframe is synchronized with every 34 cycles of this timebase signal 310.
(5) Equalizer
Equalizers used in the above-described ADSL receiver units include a time domain equalizer (TEQ) and a frequency domain equalizer (FEQ). The parallel-to-serial buffer 40 shown in FIG. 14 receives DMT symbols, each of which is represented by a simple rectangle in section (A) of FIG. 18. The buffer 40 modifies this DMT symbol, adding a copy of its last 16 samples to the beginning of the symbol, as shown in section (B) of FIG. 18. The added part is referred to as a xe2x80x9ccyclic prefix,xe2x80x9d and the DMT symbol with such a cyclic prefix is sent to the D/A converter 50 so as to be converted to an analog signal at the sampling rate of 1.104 MHz, as shown in section (C) of FIG. 18. This analog signal is transmitted to the subscriber""s premises over the metallic subscriber line 70. The signal reaching the subscriber""s premises is distorted as a result of intersymbol interference as shown in section (D) of FIG. 18. This is because of a non-ideal frequency response of the metallic subscriber line 70 which exhibits uneven amplitude and delay (phase) characteristics. Such intersymbol interference, however, is compressed within a 16-sample cyclic prefix through an equalization process provided by the TEQ unit 90 shown in FIG. 14, resulting in a waveform conceptually depicted in section (E) of FIG. 18. After that, the distorted cyclic prefix is removed at the serial-to-parallel buffer 100, thus yielding a clean DMT symbol with no effect of intersymbol interference.
In the way outlined above, the TEQ unit 90 eliminates the effect of intersymbol interference from the received signal by manipulating cyclic prefixes. More specifically, the metallic subscriber line 70 has non-linear low-pass characteristics, which cause deterioration of transmission signals at higher frequency bands. On the other hand, the data signal being transmitted shows discontinuity between adjacent symbols. This discontinuity in the data signal causes an impulse response lasting for a certain time duration, when the signal passes through the above non-linear transmission channel. (Here, the term xe2x80x9cimpulse responsexe2x80x9d refers to the waveform that results at the output of the channel when its input is excited by an impulse.) Being superimposed on the main components of the data signal, the above impulse response will lead to signal deterioration. To solve this problem, a cyclic prefix is attached to the beginning of each symbol by copying the last 16 samples of the symbol. This ensures continuity at a point where the prefix and symbol are joined, thus causing no unwanted impulse response in that part. On the other hand, the signal may still be discontinuous at the point where the beginning of the cyclic prefix is joined with the end of the previous symbol, which could cause an undesired impulse response. Fortunately, the interference at the latter part can be eliminated by (1) subjecting the received data signal to the TEQ unit 90 having a high-pass characteristic that works inversely with the low-pass characteristic of the metallic subscriber line 70, so that the impulse response will settle within the period of a cyclic prefix, and (2) removing the cyclic prefix, together with its distortion. In this way, the receiver unit decouples the received data signal from the effect of an unwanted impulse response, thus yielding its original waveform.
While the above-described TEQ unit 90 operates in the time domain, the FEQ unit 120 works in the frequency domain. That is, the FEQ unit 120 performs equalization of the decoded output of the FFT unit 110 (FIG. 14) which contains multiple carriers having different frequencies. Separately for each individual carrier, the FEQ unit 120 compensates for amplitude and phase distortions of a transmission signal, which were incurred during the travel over the metallic subscriber line 70.
(6) TEQ Training Algorithm in Frequency Domain
The TEQ unit 90 is actually a finite impulse response (FIR) filter which operates as a channel equalizer in the time domain. As shown in section (E) of FIG. 18, the TEQ unit 90 should be designed so that intersymbol interference will completely settle within a cyclic prefix having a length of sixteen sample periods, and to this end, the TEQ unit 90 has to be tuned in an adaptive manner.
Referring now to FIG. 19, an example of such adaptive algorithms for the TEQ unit 90 will be described below. While the system of FIG. 19 shares some elements with that of FIG. 14, the following explanation will focus on its distinctive points, affixing like reference numerals to like elements. Also, a series of elements from the IFFT unit 30 to the D/A converter 50 shown in FIG. 14 are now represented collectively as a single block 500 named xe2x80x9ctransmitter unitxe2x80x9d in FIG. 19. Further, while FIG. 14 represents the TEQ training block 160 as a single functional unit, FIG. 19 shows it as a collection of more specific functional blocks. They include: a reference signal generator 610, a target channel 620, and an adder 630. It should also be noted that FIG. 19 omits the A/D converter 80 and serial-to-parallel buffer 100 for simplicity of explanation.
In FIG. 19, the transmitter unit 500 is sending a signal X for use in a training process of the TEQ unit. 90. In the receiver unit 600, the same signal X is produced locally by the reference signal generator 610, which is called the reference signal X. The target channel 620 serves as a target to be referenced when training the TEQ unit 90. The adder 630 calculates the difference E between the output Z of the FFT unit 110 and the output BX of the target channel 620, and supplies the result to the TEQ unit 90 and target channel 620.
The above elements will operate as follows. The signal X transmitted by the transmitter unit 500 propagates through the metallic subscriber line 70, during which the amplitude and phase of its frequency components are varied. The TEQ unit 90 equalizes the received signal to compensate for the distortion introduced during its travel over the metallic subscriber line 70. The signal equalized as such in the time domain is then fed to the FFT unit 110 and converted into a frequency domain signal. The output of the FFT unit 110 is supplied to the adder 630. The reference signal generator 610, on the other hand, produces a reference signal X that corresponds to the transmission signal X. The target channel 620 multiplies each frequency component of the reference signal X by a predetermined coefficient. The adder 630 calculates the difference between the outputs of the FFT unit 110 and target channel 620, and feeds the result back to the TEQ unit 90 and target channel 620. Based on this difference signal, the TEQ unit 90 and target channel 620 adjust their coefficient values. More specifically, the adder 630 calculates the difference E between the output Z of the FFT unit 110 and the output BX of the target channel 620 when it is given a reference signal X, and then the TEQ unit 90 and target channel 620 are alternately adjusted in such a way that the following two conditions are both fulfilled. They are: (1) the length of the impulse response should be at most 16 sampling periods, and (2) the difference E (i.e., Zxe2x88x92BX) should be zero. As a result of this training process, the TEQ unit 90 obtains intended characteristics which confine the intersymbol interference within a 16-sample-long cyclic prefix of each received symbol, as shown in section (E) of FIG. 18. Note that the characteristic of the target channel 620 finally agrees with a combined characteristic of the metallic subscriber line 70 and TEQ unit 90.
(7) TEQ Training Algorithm in Time Domain
While FIG. 19 has illustrated an implementation of the training algorithm in the frequency domain, it is also possible to realize the same in the time domain. FIG. 20 shows a system designed to train the TEQ unit 90 in the time domain by using a generally known training algorithm. Basically, those two training systems shown in FIGS. 19 and 20 operate in the same way, although they are designed to work in different domains. Unlike its counterpart shown in FIG. 19, however, the time-domain system of FIG. 20 employs a delay unit 720, while eliminating the FFT unit 110.
The system of FIG. 20 further comprises a reference signal generator 710 and a target channel 730, both of which operate in the time domain. The reference signal generator 710 produces a time domain signal x(t) which is equivalent to what is converted from the transmission signal X. The delay unit 720 adds a predetermined amount of delay to the reference signal x(t) supplied from the reference signal generator 710. The target channel 730 is used as a target to be referenced when tuning the characteristic of the TEQ unit 90. It outputs a signal b(t)*x(t), where the function b(t) represents the characteristics of the target channel 730 itself, and the asterisk (*) denotes the convolution operator. This means that the target channel 730 calculates a convolution integral of the delayed reference signal x(t) with the target transmission characteristics b(t). The adder 630 calculates the difference between the output z(t) of the TEQ unit 90 and the output b(t)*x(t) of the target channel 730. The result is fed back to the TEQ unit 90 and target channel 730. Based on this difference signal, the TEQ unit 90 and target channel 730 adjust themselves in such a way that the error output e(t) of the adder 630 will be zero.
The above-described conventional system operates as follows. When a signal X is transmitted at the sending end, its frequency components are varied during the transport over the metallic line 70. The TEQ unit 90 thus performs equalization of the received signal to compensate for the distortion. The resultant signal z(t) is then fed to the adder 630. On the other hand, the reference signal generator 710 produces a time-domain reference signal x(t), which is equivalent to the transmission signal X. The delay unit 720 delays this reference signal x(t) by a predetermined time interval, such that the output z(t) of the TEQ unit 90 will be in phase with that of the target channel 730. The target channel 730 convolves the reference signal x(t) with a combined characteristic b(t) of the metallic subscriber line 70 and TEQ unit 90 (excluding the delay time of the metallic subscriber line 70). The result of this convolution integral b(t)*x(t) is then supplied to the adder 630. The adder 630 calculates the difference e(t) between the output z(t) of the TEQ unit 90 and the output b(t)*x(t) of the target channel 730. The result is fed back to the TEQ unit 90 and target channel 730. Based on this result, the TEQ unit 90 and target channel 730 adjust themselves in an adaptive way. That is, the TEQ unit 90 and target channel 730 perform training operations alternately so that the error output e(t) of the adder 630 will be zero. As a result of this process, the TEQ unit 90 obtains intended characteristics, thus confining the intersymbol interference within a 16-sample-long cyclic prefix of each received symbol, as shown in section (E) of FIG. 18.
(8) FEQ Training Algorithm
Referring now to FIG. 21, an example of FEQ training algorithms will be described below. FEQ unit is an equalizer operating in the frequency domain. Since the DMT modulation uses multitone carriers, it is actually configured as a collection of equalizers for individual carriers having different frequencies in such a way that uniform characteristics will be provided over various tones.
FIG. 21 shows a combination of an FEQ unit 840 and a decoder 850, which correspond to the FEQ unit 120 and decoder 130 in FIG. 14, respectively. The FEQ unit 840 comprises a multiplier 810 which multiplies a given input signal Yi by a coefficient Wi. The values of the coefficients Wi are varied depending on the output of the adder 830. The decoder 850 comprises a decision unit 820 which outputs an estimated value Xi by guessing what the input signal Yi is likely to be. The adder 830 calculates the difference between the output Xi of the decision unit 820 and the output Zi of the multiplier 810, and based on the difference value, it determines the coefficients Wi for the multiplier 810.
The above conventional equalizer operates as follows. Multiple carrier signals are modulated by the transmitter unit and transmitted over the metallic subscriber line 70. When they reach the receiving end, those carrier signals exhibit some distortion in their amplitude and phase values because of the non-ideal characteristics of the metallic subscriber line 70. The FEQ unit 120 compensates for such distortion of each individual carrier signal. To this end, the FEQ unit 120 provides as many equalization circuits of FIG. 21 as the number of carriers, and each such circuit employs its local decoder 130 to produce an estimated value and compensate for the deterioration in a particular carrier frequency band. That is, in order for the FEQ unit 120 to compensate for the amplitude and phase distortion, each individual equalization circuit (FIG. 21) has to work in an adequate manner in its own carrier frequency band. To this end, the circuit is configured as follows: (1) each FEQ unit 840 provides its output Zi to the corresponding decoder 850, thus yielding an estimated value Xi; (2) the adder 830 calculates the difference Ei between the estimated value Xi and FEQ output Zi; and (3) the coefficients Wi of the multiplier 810 are controlled so that the difference Ei will be zero. The decoder 850 further converts the estimated value Xi into data bits bi and supplies them to the parallel-to-serial buffer 140 (FIG. 14).
(9) Frame Synchronization
FIG. 22 outlines an initialization sequence to start up an ADSL transmission unit. As indicated in FIG. 22, signals transmitted at this initialization stage 900 include continuous signals 910 and discontinuous signals 920. More specifically, in the first half of the initialization stage 900, the ADSL unit transmits continuous signals 910 such as repetitive synchronization symbols, and in the second half, it transmits discontinuous signals 920 such as wideband synchronization symbols.
When sending discontinuous signals, it is necessary for the transmitter unit to add a cyclic prefix to each DMT symbol to send, to enable the receiver unit to eliminate the effect of intersymbol interference from the received DMT symbols, as shown in section (C) of FIG. 18. However, when sending continuous signals, the transmitter unit can use DMT symbols with no cyclic prefix as shown in section (A) of FIG. 18. That is, there is no need to add a cyclic prefix to DMT symbols in the latter case, because continuous signals do not suffer from intersymbol interference.
The receiver unit has to synchronize itself to the timing of transmission frames to identify and extract each incoming symbol correctly from the received signal. When a continuous signal is being transmitted, the receiver unit can identify DMT symbols at any phase. In the case of discontinuous signals, however, correct DMT symbols can be identified only at one particular phase. Frame synchronization should therefore be established during an initialization stage where the transmission signal is continuous, so that the receiver unit will be able to correctly receive DMT symbols in a later communication session. More specifically, the receiver unit first captures the phase of DMT symbols at an initialization stage during which a continuousness transmission signal is provided, as shown section (A) of FIG. 23. It then attempts frame synchronization by shifting the captured phase as shown in section (B) of FIG. 23, so that DMT symbols can be extracted from discontinuous signals.
As FIG. 22 shows, the frame synchronization is performed after the TEQ training is completed, for the following reason. The TEQ unit adds some amount of delay, and this delay should also be considered as one of the parameters that affect the frame synchronization.
(10) Equalizer Training in TCM Crosstalk-prevalent Environment
For correct transport of ADSL signals, it is desired that both the aforementioned TEQ unit 90 and FEQ unit 120 are optimized in such environments where TCM crosstalk interference is prevalent, whether the system is in an initialization stage or in a normal communication session. One example of training algorithms for TEQ and FEQ units at an initialization stage in TCM crosstalk-prevalent environments is proposed in Japanese Patent Application No. 10-172464 (1998). According to this patent application, when performing equalizer training, the receiver unit refers to inside symbols in FEXT bit map mode (single bit map mode), whereas it refers only to inside symbols, or successively refers to both inside and outside symbols in dual bit map mode. In the case that both inside and outside symbols are used in dual bit map mode, the coefficient updating step size for outside symbols should be set to the vicinity of zero (i.e., zero or a sufficiently small value).
As clarified above, the time domain equalizer has to be adequately characterized through a training process, so that the intersymbol interference will be confined within a 16-sample-long cyclic prefix. However, the equalizer could lose its performance during a long time operation because of changes in temperature and consequent variations in the characteristics of a metallic subscriber line being used. Thus, to maintain the optimal performance during a normal data communication session, it is necessary to continue some processing to adapt the equalizer even after the initialization sequence is finished. However, no practical methods have been proposed so far to meet this requirement.
Another problem is that the training process in the initialization sequence consumes a large amount of processing resources and time, while the equalizer training is an essential process to obtain a good TEQ characteristic.
Still another problem with the conventional systems is the lack of training algorithms in an environment where TCM crosstalk interference is prevalent. Although some algorithms for use in the initialization phase are available, as described earlier in item (10), no practical algorithms or implementation methods have been proposed for how to train the equalizer during normal data communication sessions.
Taking the above into consideration, an object of the present invention is to provide a data transmission system which reliably trains its equalizers both in an initialization phase and in a normal communication session.
To accomplish the above object, according to the present invention, there is provided a data transmission system which uses a subscriber line to transport data from a transmitter unit to a receiver unit. In this system, the transmitter unit comprises the following elements: a modulator which applies a prescribed modulation on a data signal to be transmitted; a prefixing unit which adds a cyclic prefix to the data signal modulated by the modulator; a transmitter which sends the data signal with the cyclic prefix over the subscriber line. The receiver unit, on the other hand, comprises the following element: a receiver which receives the data signal sent from the transmitter; a processor which processes the data signal received by the receiver so that impairment introduced in the received data signal will be confined within the cyclic prefix, where the impairment has been introduced during the transport of the data signal over the subscriber line; a target channel which provides reference characteristics for use in training the processor; a first training unit which performs training at an initialization stage by tuning both of the target channel and the processor; and a second training unit which performs training by tuning the processor when a normal communication session takes place.
Further, another object of the present invention is to provide a receiver unit which maintains the performance of equalizers with reduced processing time and loads.
To accomplish this second object, according to the present invention, there is provided a receiver unit which receives data sent from a transmitter unit over a subscriber line. This receiver unit comprises the following elements: a receiver which receives a data signal sent from the transmitter unit; a processor which processes the data signal received by the receiver so that impairment introduced in the received data signal will be confined within a cyclic prefix, where the impairment has been introduced during the transport of the data signal over the subscriber line; a target channel which provides reference characteristics for use in training the processor; a first training unit which performs training at an initialization stage by tuning both of the target channel and the processor; and a second training unit which performs training by tuning the processor when a normal communication session takes place.
The above and other objects, features and advantages of the present invention will become apparent from the following description when taken in conjunction with the accompanying drawings which illustrate preferred embodiments of the present invention by way of example.